KR102377657B1 - Mullite-containing sintered body, method for manufacturing the same, and composite substrate - Google Patents

Abstract

The mullite-containing sintered compact of the present invention contains at least one selected from the group consisting of silicon nitride, silicon oxynitride and sialon in addition to mullite. The mullite-containing sintered compact preferably has a thermal expansion coefficient of less than 4.3 ppm/°C at 40°C to 400°C, an open porosity of 0.5% or less, and an average grain size of 1.5 µm or less.

Description

Mullite-containing sintered compact, manufacturing method thereof, and composite substrate

The present invention relates to a mullite-containing sintered body, a manufacturing method thereof, and a composite substrate.

In general, the mullite sintered body is a material excellent in thermal shock resistance obtained by sintering aluminum oxide (Al ₂ O ₃ ) and silicon oxide (SiO ₂ ) in a ratio of 3:2, and is represented by 3Al ₂ O ₃ ·2SiO ₂ . As such a mullite sintered compact, as disclosed in Patent Document 1, for example, a powder obtained by mixing 30% by mass of yttrium oxide stabilized zirconia (YSZ) powder with mullite powder is molded and the compact is sintered. In Patent Document 1, a mullite substrate is cut out from a mullite sintered body and the main surface of the mullite substrate is polished to obtain a base substrate used for bonding to a GaN substrate. The coefficient of thermal expansion of GaN is 6.0 ppm/K in the range from room temperature to 1000°C, and the coefficient of thermal expansion of mullite is 5.2 ppm/K. Therefore, considering the bonding of both, it is desirable to increase the thermal expansion coefficient of mullite to approximate that of a GaN substrate.

On the other hand, Patent Document 2 describes an example in which a composite substrate in which a functional substrate made of lithium tantalate or lithium niobate and a support substrate made of a mullite sintered body are joined by direct bonding is used for a surface acoustic wave device such as a surface acoustic wave device. has been In such an elastic wave device, the thermal expansion coefficient of the mullite substrate, which is the supporting substrate, is as small as 4.4 ppm/°C (40 to 400°C) and the Young's modulus is as large as 220 GPa or more. and, accordingly, the temperature dependence of the frequency is greatly improved. In order to directly bond the functional substrate and the supporting substrate, high flatness is required at each bonding surface. For example, it is described in patent document 2 that it is preferable that center line average roughness Ra is 3 nm or less.

[Patent Document 1] Japanese Patent No. 5585570 [Patent Document 2] Japanese Patent No. 5861016

However, Patent Document 1 describes a mullite sintered compact with a high coefficient of thermal expansion by adding a significant amount of other components to mullite, and Patent Document 2 describes a mullite sintered compact with high mullite purity. There is no mention of a reduced mullite sintered compact, and furthermore, as such a low thermal expansion mullite sintered compact, it is not known that the polished surface has a high surface flatness. Further, even with low thermal expansion, when a mullite sintered body with low rigidity is used as a support substrate for a composite substrate, the composite substrate may warp due to a slight temperature difference.

The present invention has been made in order to solve these problems, and in a sintered body containing mullite, the main object is to have a low coefficient of thermal expansion and high rigidity as compared to mullite alone, and to increase the flatness of the polished surface.

The mullite-containing sintered body of the present invention is a mullite-containing sintered body containing at least one selected from the group consisting of silicon nitride, silicon oxynitride, and sialon in addition to mullite, and has a thermal expansion coefficient of 40 to 400° C. of less than 4.3 ppm/° C. , the open porosity is 0.5% or less, and the average crystal grain size (average particle size of the sintered particles) is 1.5 µm or less. This mullite-containing sintered compact has a lower coefficient of thermal expansion and higher rigidity than mullite alone. Moreover, the flatness of a grinding|polishing surface can be made high.

In the manufacturing method of the mullite-containing sintered body of the present invention, (a) 50 to 90% by volume of mullite powder having an average particle diameter of 1.5 μm or less and 10 to 50% by volume of silicon nitride powder having an average particle diameter of 1 μm or less are mixed so that a total of 100% by volume to obtain a mixed raw material powder; (b) molding the mixed raw material powder into a molded body having a predetermined shape, and hot press firing the molded body at a press pressure of 20 to 300 kgf/cm 2 and a firing temperature of 1525 to 1700 ° C. By doing so, the process of obtaining a mullite containing sintered compact is included. This production method is suitable for producing the mullite-containing sintered body of the present invention described above. In addition, the average particle diameter of a powder is the value measured by the laser diffraction method (it is the same hereafter).

The composite substrate of the present invention is a composite substrate in which a functional substrate and a supporting substrate are bonded, and the supporting substrate is the above-described mullite-containing sintered body. Since this composite substrate has a high flatness of the polished surface of the mullite-containing sintered body serving as the supporting substrate, it is well bonded to the functional substrate. In addition, when this composite substrate is used for a surface acoustic wave device, the frequency-temperature dependence is greatly improved. Moreover, also in an optical waveguide device, an LED device, and a switch device, the performance improves because the thermal expansion coefficient of a support substrate is small.

1 is a manufacturing process diagram of a mullite-containing sintered body.
2 is a perspective view of a composite substrate 10;
3 is a perspective view of an electronic device 30 manufactured using a composite substrate 10;

Hereinafter, although embodiment of this invention was described concretely, this invention is not limited to the following embodiment, It is a range which does not deviate from the meaning of this invention, Based on common knowledge of those skilled in the art, appropriate change and improvement It should be understood that the back is applied.

The mullite-containing sintered compact of this embodiment contains at least 1 sort(s) selected from the group which consists of silicon nitride, silicon oxynitride, and sialon other than mullite. It is preferable that the mullite is the component (main component) contained the most in the sintered body, but the component selected from the above group may be the main component. It is preferable that this mullite-containing sintered compact has a thermal expansion coefficient of less than 4.3 ppm/°C at 40 to 400°C, an open porosity of 0.5% or less, and an average grain size of 1.5 µm or less. This mullite-containing sintered compact has a lower coefficient of thermal expansion and a higher Young's modulus (rigidity) than mullite alone. In addition, since this mullite-containing sintered body has an open porosity of 0.5% or less, almost no pores, and a small average grain size of 1.5 µm or less, the flatness of the polished surface (polished surface) is improved.

In the mullite-containing sintered body of the present embodiment, it is preferable that the number of pores having a maximum length of 1 µm or more with respect to an area of 100 µm×100 µm of the polished surface is 10 or less. When the number of pores is 10 or less, the flatness of the polished surface becomes higher. The number of such pores is more preferably three or less, and still more preferably zero.

It is preferable that the Young's modulus of the mullite-containing sintered compact of this embodiment is 240 GPa or more, and it is preferable that four-point bending strength is 300 MPa or more. Since silicon nitride or components derived therefrom have higher Young's modulus and strength than mullite, by adjusting the addition ratio of silicon nitride to mullite, the Young's modulus of the mullite-containing sintered body is set to 240 GPa or higher, or the four-point bending strength is set to 300 MPa or higher. or you can Moreover, as for 4-point bending strength, it is more preferable that it is 320 MPa or more.

In the mullite-containing sintered body of the present embodiment, it is preferable that the center line average roughness Ra of the polished surface is 1.5 nm or less. It is known that a functional substrate and a supporting substrate are bonded as a composite substrate for use in acoustic wave devices. However, by using a mullite-containing sintered body having a polished surface Ra of 1.5 nm or less as a supporting substrate as described above, bonding between the supporting substrate and the functional substrate is improved. get better For example, the ratio (junction area ratio) of the area actually joined among the bonding interfaces is 80% or more (preferably 90% or more). The center line average roughness Ra of the polished surface is more preferably 1.1 nm or less, and still more preferably 1.0 nm or less.

As for the mullite containing sintered compact of this embodiment, it is more preferable that the thermal expansion coefficient of 40-400 degreeC is 3.8 ppm/degreeC or less. By using a composite substrate having such a mullite-containing sintered body as a support substrate for an elastic wave device, the functional substrate undergoes a smaller thermal expansion than the original thermal expansion when the temperature of the elastic device rises, so that the frequency-temperature dependence of the elastic device is improved. It is more preferable that the thermal expansion coefficient of 40-400 degreeC is 3.5 ppm/degreeC or less.

Next, one Embodiment of the manufacturing method of the mullite containing sintered compact of this invention is demonstrated. The production flow of the mullite-containing sintered body includes (a) a step of preparing the mixed raw material powder and (b) a step of producing the mullite-containing sintered body, as shown in FIG. 1 .

·Step (a): Preparation of mixed raw material powder

The mixed raw material powder is prepared by mixing mullite powder and silicon nitride powder. As the mullite raw material, it is preferable to use a powder having high purity and a small average particle size. The purity is preferably 99.0% or more, more preferably 99.5% or more, and still more preferably 99.8% or more. The unit of purity is mass %. Moreover, 1.5 micrometers or less are preferable and, as for the average particle diameter (D50), 0.1-1.5 micrometers is more preferable. As a mullite raw material, a commercially available item may be used and what was produced using high-purity alumina or silica powder may be used for it. As a method of producing a mullite raw material, the method described in patent document 2 is mentioned, for example. As the silicon nitride raw material, it is preferable to use a powder having a small average particle size. 1 micrometer or less is preferable and, as for an average particle diameter, 0.1-1 micrometer is more preferable. The mixing ratio of the mullite raw material and the silicon nitride raw material is, for example, 50 to 90 vol% (preferably 70 to 90 vol%) of the mullite raw material and 10 to 50 vol% (preferably 10 to 30 vol%) of the silicon nitride raw material. It may be weighed so as to be 100% by volume in total, mixed with a mixer such as a pot mill, and, if necessary, dried with a spray dryer to obtain a mixed raw material powder.

·Step (b): Preparation of mullite-containing sintered body

The mixed raw material powder obtained in step (a) is molded into a molded body having a predetermined shape. There is no restriction|limiting in particular in the shaping|molding method, A general shaping|molding method can be used. For example, the mixed raw material powder may be press-molded with a mold as it is. In the case of press molding, when the mixed raw material powder is made into granular form by spray drying, the moldability is improved. In addition, an organic binder may be added to prepare a clay to be extruded, or a slurry may be prepared to form a sheet. In these processes, it becomes necessary to remove the organic binder component before or during the firing process. In addition, high pressure molding may be performed by CIP (Cold Hydrostatic Press).

Next, the obtained molded body is fired to produce a mullite-containing sintered body. At this time, it is preferable in order to improve the surface flatness of the mullite-containing sintered body to keep the sintered particles fine and to discharge gas during sintering. As the method, the hot press method is very effective. By using this hot press method, densification proceeds in the state of fine particles at a low temperature compared to normal pressure sintering, and it is possible to suppress the residual coarse pores frequently seen in atmospheric sintering. As for the firing temperature (maximum temperature) at the time of this hot press, 1525-1700 degreeC is preferable. In addition, it is preferable that the press pressure at the time of hot press shall be 20-300 kgf/cm<2>. In particular, at a low press pressure, since miniaturization and long life of a hot press jig|tool are possible, it is preferable. The holding time at the firing temperature can be appropriately selected in consideration of the shape and size of the molded article, the characteristics of the heating furnace, and the like. A specific preferable holding time is, for example, 1 to 12 hours, more preferably 2 to 8 hours. The firing atmosphere is not particularly limited, and an inert atmosphere such as nitrogen or argon is generally used as the atmosphere at the time of hot pressing. The temperature increase rate and temperature decrease rate may be appropriately set in consideration of the shape and size of the molded article, the characteristics of the heating furnace, and the like, and may be set, for example, in the range of 50 to 300°C/hr.

Next, an embodiment of the composite substrate of the present invention will be described. In the composite substrate of the present embodiment, the functional substrate and the support substrate for manufacturing the mullite-containing sintered body described above are bonded together. This composite substrate has a large bonding area ratio between the two substrates, and exhibits good bonding properties. Although it does not specifically limit as a functional board|substrate, For example, lithium tantalate, lithium niobate, gallium nitride, silicon, etc. are mentioned. As for the bonding method, direct bonding is preferable. In the case of direct bonding, the respective bonding surfaces of the functional substrate and the supporting substrate are polished and then activated, and both substrates are pressed while the bonding surfaces are facing each other. Activation of the bonding surface is performed, for example, by irradiation with an ion beam of an inert gas (argon or the like) to the bonding surface, but also by irradiation with plasma or a neutral atom beam. It is preferable that the ratio (thickness of a functional substrate/thickness of a support substrate) of the thickness of a functional substrate and a support substrate is 0.1 or less. Fig. 2 shows an example of a composite substrate. The composite substrate 10 is one in which a piezoelectric substrate 12 as a functional substrate and a support substrate 14 are directly bonded by bonding.

The composite substrate of the present embodiment can be used for electronic devices and the like. Examples of such electronic devices include LED devices, optical waveguide devices, and switch devices in addition to acoustic wave devices (such as surface acoustic wave devices, Ram wave elements, and thin film resonators (FBARs)). When the above-described composite substrate is used for the acoustic wave device, the coefficient of thermal expansion of the mullite-containing sintered body as the supporting substrate is as small as 4.3 ppm/K (40 to 400° C.), so that the frequency-temperature dependence is greatly improved. An example of the electronic device 30 manufactured using the composite board|substrate 10 in FIG. 3 is shown. The electronic device 30 is a one-port SAW resonator, that is, a surface acoustic wave device. First, patterns of a plurality of electronic devices 30 are formed on a piezoelectric substrate 12 of a composite substrate 10 using a general photolithography technique, and then, each electronic device 30 is formed by dicing. Cut out. The electronic device 30 has IDT (Interdigital Transducer) electrodes 32 and 34 and a reflective electrode 36 formed on the surface of the piezoelectric substrate 12 by photolithography.

In addition, this invention is not limited at all to the above-mentioned embodiment, It goes without saying that it can be implemented in various aspects as long as it falls within the technical scope of this invention.

Example

1. Preparation of mixed raw material powder

As the mullite raw material, commercially available mullite powder having a purity of 99.9% or more and an average particle diameter of 1.5 µm was used, and a commercially available silicon nitride powder having a purity of 97% or more and an average particle diameter of 0.8 µm was used as a silicon nitride raw material. The mullite raw material and the silicon nitride raw material were weighed in the proportions shown in Experimental Examples 1 to 3 in Table 1, and pot mill-mixed using alumina aldol having a diameter of 5 mm, and spray-drying to prepare a mixed raw material powder.

*1: The peak area of the mullite (210) plane (2θ = 26.2°) of the XRD profile is set to 1, and the peak area of each crystal phase to it is taken as the crystal phase ratio

*2: From the IR transmission image, a junction area ratio of 90% or more is considered "best", and a junction area ratio of 80% or more and less than 90% is considered "good".

2. Fabrication of mullite-containing sintered compact

The mixed raw material powders of Experimental Examples 1 to 3 were put into a mold with a diameter of about 125 mm, and molded into a disk shape with a thickness of about 10 to 15 mm at a pressure of 200 kgf/cm 2 to obtain a mullite-containing molded body. Then, the mullite-containing molded body was placed in a graphite mold for hot pressing with an inner diameter of about 125 mm, and a mullite-containing sintered body having a diameter of about 125 mm and a thickness of about 5 to 8 mm was produced by hot pressing. In addition, the maximum temperature (calcination temperature) at the time of baking was 1650 degreeC, the holding time at the calcination temperature was 5 hr, and both the temperature increase rate and temperature fall temperature were 100 degreeC/hr. The press load was set to 200 kgf/cm 2 at 900° C. or higher during temperature rise, and the furnace atmosphere was vacuumed up to 900° C. After reaching 900° C., N ₂ was introduced to proceed with sintering under N ₂ . After holding the calcination temperature for a predetermined time, the temperature was lowered to 1200°C, the press load and control of the furnace atmosphere were stopped, and the temperature was naturally cooled to room temperature. In addition, in Experimental Example 4, a molded body and a sintered body were produced in the same manner only with the mullite powder.

3. Characteristic evaluation

From the sintered body of Experimental Examples 1 to 4, a test piece (such as a 4×3×40 mm size anti-cutting rod) was cut out, and various characteristics were evaluated. In addition, the polished surface of the sintered compact was made to be mirror-finished by grinding|polishing one surface of the test piece of about 4x3x10 mm. Polishing was performed in the order of diamond abrasive grains of 3 µm and diamond abrasive grains of 0.5 µm, and lab polishing was performed using diamond abrasive grains of 0.1 µm or less for final finishing. The evaluated characteristics are as follows.

(1) crystalline phase

The sintered compact was pulverized, and the crystal phase was identified by an X-ray diffraction apparatus. Measurement conditions were CuKα, 50 kV, 300 mA, 2θ = 5-70°, and a rotating counter-cathode type X-ray diffractometer (RINT, manufactured by Rhythmic Electric Co., Ltd.) was used.

(2) crystalline phase ratio

From the X-ray diffraction profile of (1) above, the peak area ratio of each crystal phase was calculated. The peak area of the mullite (210) plane (2θ = 26.2°) was set to 1, and the peak area of each crystal phase to it was defined as the crystal phase ratio. Here, as a representative peak of each crystal phase, silicon nitride has a (101) plane (2θ = 20.6°), sialon is a (3-20) plane of Si ₂ Al ₃ O ₇ N (2θ = 24.6°) and Si ₅ AlON ₇ A (200) plane (2θ = 26.9°) was used.

(3) bulk density, open porosity

Bulk density and open porosity were measured by Archimedes' method using pure water using a cleavage rod.

(4) Young's modulus

It measured by the static bending method according to JIS R 1602. The shape of the test piece was made into a 3 mm x 4 mm x 40 mm cross-section bar.

(5) bending strength

According to JIS R1601, the four-point bending strength was measured. The shape of the test piece was made into a 3 mm x 4 mm x 40 mm cross-section bar or its half size.

(6) Coefficient of thermal expansion (40~400℃)

According to JIS R1618, it was measured by the pressure rod differential method. The test piece shape was 3 mm x 4 mm x 20 mm.

(7) Number of pores

The polished surface of the sintered body finished as described above was observed by SEM, and the number of pores having a maximum length of 1 μm or more per 100 μm×100 μm was counted.

(8) Surface flatness (Ra)

With respect to the polished surface of the sintered body finished as described above, the center line average roughness Ra was measured using AFM. The measurement range was 10 µm×10 µm.

(9) Average particle size of sintered particles

The polished surface of the sintered body finished as described above was chemically etched with phosphoric acid, the size of 200 or more sintered particles was measured by SEM, and the average particle diameter was calculated using a line segmentation method. The coefficient of the line segmentation method was set to 1.5, and the value obtained by multiplying the length measured by SEM by 1.5 was used as the average particle diameter.

(10) Adhesion

A disk having a diameter of 100 mm and a thickness of about 600 µm was cut out from the sintered bodies of Experimental Examples 1 to 4. After the original plate was polished as described above, it was washed to remove particles and contaminants from the surface. Next, using this original plate as a support substrate, direct bonding between the support substrate and the functional substrate was performed to obtain a composite substrate. That is, first, each bonding surface of the support substrate and the functional substrate was activated by an ion beam of argon, and then both bonding surfaces were faced to each other and pressed with 10 tonf, followed by bonding to obtain a composite substrate. As the functional substrate, a lithium niobate (LN) substrate was used. In the evaluation of bonding properties, from the IR transmission image, those having a bonding area ratio of 90% or more were “best”, those of 80% or more and less than 90% were “good”, and those of less than 80% were “poor”.

4. Evaluation Results

In the mullite-containing sintered compacts of Experimental Examples 1 to 3, a mixed raw material powder obtained by mixing a mullite raw material and a silicon nitride raw material was calcined, but a part of silicon nitride was changed to sialon by calcination. Since the mullite-containing sintered compacts of Experimental Examples 1 to 3 contained silicon nitride and the like, the Young's modulus and 4-point bending strength were improved compared to the mullite-only sintered compact of Experimental Example 4 . That is, the Young's modulus was improved to 240 GPa or more, and the four-point bending strength was improved to 320 MPa or more. In addition, the mullite-containing sintered body of Experimental Examples 1 to 3 had a coefficient of thermal expansion of 40 to 400°C of less than 4.3 ppm/°C (3.5-4.1 ppm/°C), which was lower than that of the mullite-only sintered body of Experimental Example 4 became In addition, the mullite-containing sintered body of Experimental Examples 1 to 3 or the mullite alone sintered body of Experimental Example 4 had an open porosity of 0.5% or less (less than 0.1%) and an average grain size of 1.5 μm or less (1.0-1.2 μm). Therefore, the center average roughness Ra of the polished surface was reduced to 1.1 nm or less (0.9 to 1.1 nm). Therefore, the bondability when the original plate cut out from the sintered body of Experimental Examples 2 to 4 is directly bonded to the functional substrate is "best" in all of the bonding area ratios of 90% or more, and the original plate cut out from the sintered body of Experimental Example 1 is used as a functional substrate. The bondability when directly joined to the joint was "good" with a joint area ratio of 80% or more and less than 90%. In addition, the fact that the number of pores is three or less (zero) also contributes to such a small value of the central average roughness Ra of the polished surface.

Further, Experimental Examples 1 to 3 correspond to Examples of the present invention, and Experimental Example 4 corresponds to Comparative Examples. These experimental examples do not limit the present invention at all.

This application is based on the priority claim of Japanese Patent Application No. 2016-058970 for which it applied on March 23, 2016, The whole content is incorporated in this specification by reference.

10: composite substrate 12: piezoelectric substrate
14: support substrate 30: electronic device
32, 34: IDT electrode 36: reflective electrode

Claims (10)

In the mullite-containing sintered body containing at least one selected from the group consisting of silicon nitride, silicon oxynitride and sialon in addition to mullite,
The coefficient of thermal expansion at 40°C to 400°C is 4.1 ppm/°C or less,
The open porosity is 0.5% or less,
an average grain size of 1.5 μm or less,
A mullite-containing sintered body, wherein the average roughness Ra of the center line of the polished surface is 1.5 nm or less. The mullite-containing sintered body according to claim 1, wherein the number of pores having a maximum length of 1 μm or more existing for an area of 100 μm×100 μm of the polished surface is 10 or less. The mullite-containing sintered body according to claim 1 or 2, wherein the Young's modulus is 240 GPa or more. The mullite-containing sintered body according to claim 1 or 2, wherein the four-point bending strength is 320 MPa or more. delete delete The mullite-containing sintered body according to claim 1 or 2, wherein the four-point bending strength is 350 MPa or less. In the manufacturing method of a mullite-containing sintered compact,
(a) Mixing 50% to 90% by volume of a mullite powder having an average particle diameter of 1.5 μm or less and 10% to 50% by volume of a silicon nitride powder having an average particle diameter of 1 μm or less to a total of 100% by volume to obtain a mixed raw material powder class,
(b) The mixed raw material powder is molded into a molded body having a predetermined shape, and the molded body is subjected to hot press firing at a press pressure of 20 kgf/cm 2 to 300 kgf/cm 2 and a firing temperature of 1525° C. to 1700° C. The process of obtaining the mullite-containing sintered body described in
A manufacturing method of a mullite-containing sintered body comprising a. 9. The method of claim 8,
The temperature increase rate is 50°C/hr to 300°C/hr,
A method for producing a mullite-containing sintered body, wherein the holding time at the calcination temperature is 2 to 8 hours. A composite substrate in which a functional substrate and a support substrate are bonded, the composite substrate comprising:
The support substrate is a composite substrate that is a mullite-containing sintered body according to claim 1 or 2.

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